1. Field of the Invention
The present invention generally relates to an image forming apparatus, and, in particular, to an image forming apparatus which performs control of saving/reducing developing agent by PWM modulation, for example, depending on input image data, output resolution, a mode of image quality, and so forth, so as to effectively save/reduce a consumption of developing agent while maintaining a high image quality.
2. Description of the Related Art
In an image forming apparatus in an electrophotographic type such as a laser printer, a digital copier, a ordinary-paper facsimile apparatus and so forth, various methods of reducing/saving a consumption of developing agent (which will be simply referred to as a toner, hereinafter) such as a toner have been proposed.
For example, Japanese Laid-Open Patent Application No. 7-111588 discloses a method of adjusting the number of pixels printed out in a bitmap form transformed into raster. According to this method, a bit mask having a pattern including active pixels and inactive pixels, and, a logical AND performed between a bitmap transformed into raster and the above-mentioned bit mask is used. Thus, the number of pixels which are printed out is reduced, and, as a result, the amount of toner used (toner consumption) is reduced.
Accordingly, by this method, the bit mask is overlaid on the bitmap, and then, a logical AND therebetween is obtained. Thereby, the number of pixels actually printed out is reduced so that the toner consumption is reduced.
Further, a Japanese Laid-Open Patent Application No. 7-228005 discloses a printing apparatus which prints out data transmitted from a host computer. This apparatus has means for giving instructions of altering a recording density, and recording control means for altering the recording density of each pixel into a predetermined recording density according to the instructions. Thereby, in this printing apparatus, the toner consumption is reduced, and, also, each dot is positively reproduced, by reducing a total interval for which a laser is turned on for each pixel through PWM (pulse width modulation).
However, in the above-mentioned method of Japanese Laid-Open Patent Application No. 7-111588, the density of pixels of a digital image actually printed out is adjusted. Thereby, it is possible to obtain a stable output of black pixels regardless of process conditions, in comparison to a method in which the toner consumption is reduced through reduction in toner thickness. However, as the image data itself is reduced thereby, the image quality is degraded for each dot.
In the above-mentioned apparatus of Japanese Laid-Open Patent Application No. 7-228005, as shown in FIGS. 1A and 1B, each pixel having a pixel value more than 0 is converted into a predetermined uniform PWM pattern, regardless of the specific pixel value. Thereby, even a zone having minute dots scattered as a noise, or a very light tone zone becomes the same as a dark tone zone. Accordingly, even in a mode of toner saving in which an image quality is not important, or a draft mode, an undesirable image may be output. Further, as a light tone zone is converted by using a same PWM pattern as that of a dark tone zone, the toner may be used even more than a case of not employing this conversion.
An object of the present invention is to provide an image forming apparatus by which, while an image quality is maintained from being degraded much, saving of a developing agent can be effectively achieved.
An image forming apparatus comprises:
an image forming part which forms a visible image based on image data for each pixel by using a developing agent; and
an image converting part which outputs a predetermined developing agent saving pattern for a pixel having a pixel value exceeding a predetermined threshold, but performing no output for a pixel having a pixel value not larger than the predetermined threshold.
Thereby, no output is made for each pixel having a pixel value/image data not larger than the predetermined threshold, and, as a result, no developing agent is made adhere to a photosensitive body for the pixel. Accordingly, it is possible to prevent an excessive output from being made for a minute dot, and, also, to reduce the consumption of the developing agent effectively, while image quality in a thus-formed image is maintained at a still satisfactory level. Further, for each pixel having a pixel value/image data larger than the threshold value, the developing agent saving pattern is output uniformly. Thereby, some dots are thinned out, as shown in FIG. 1D for each pixel, and, thus, the saving of the developing agent is also performed effectively, without much degrading the image quality.
The image forming part may form an electrostatic latent image on a photosensitive body by applying light thereon from a light source based on the image data, and develop the electrostatic latent image by using the developing agent; and
the developing agent saving pattern may comprise a PWM pattern turning on/off the light source several times for one pixel.
Thereby, as mentioned above, as shown in FIG. 1D, some dots are thinned out for each dot according to a predetermined pattern, and, thus, saving of the consumption of the developing agent may be effectively achieved.
The predetermined threshold may be altered in accordance with an image quality of an image to be formed.
Thereby, it is possible to perform appropriate output for a minute dot which is special for each image quality which is set, thus, to appropriately reduce the consumption of the developing agent, while rendering a more satisfactory image quality.
The predetermined threshold may be altered in accordance with a resolution of an image to be formed.
Thereby, it is possible that no output is made selectively for a light tone zone of an image for which output is otherwise made at a relatively dark tone when a low resolution is set, and, thus, it is possible to appropriately reduce the consumption of the developing agent, while rendering a more satisfactory image quality.
The image forming apparatus may further include a setting part by which a developing agent saving function is set on/off, and
wherein the image data converting part functions only when the developing agent saving function is set on, but, otherwise, conversion of image data is not performed by the image data converting part.
Thereby, according to a request of a user, a high-quality image may be output, or saving of the consumption of the developing agent may be performed effectively.
The developing agent saving function may be made to be set off as a result of the above-mentioned predetermined threshold being set to be xe2x80x9c0xe2x80x9d.
Thereby, it is possible to omit a special circuit for controlling on/off of the developing agent saving function, thus, to provide an inexpensive image forming apparatus by which the consumption of the developing agent can be performed effectively.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.